Steam Cracker Co-Processing Mass Balance Basics
Co-processing plastic pyrolysis oil in steam crackers requires yield-based mass balance attribution under ISCC PLUS rules to substantiate circular claims.
Silicon Contamination
Meaning
The presence of silicon-containing compounds in polymer feedstocks degrades catalyst performance and introduces defects into finished moulded articles. Often originating from silicone-based additives, polydimethylsiloxanes or contamination during post-consumer waste sorting, silicon contamination poses a severe threat to both chemical recycling and melt processing. This impurity alters the surface tension and mechanical properties of the affected resin, even at part-per-million levels.
The boundary of this contamination’s impact is measured from the primary feedstock intake of the cracker to the final injection-moulded part.
Source Origin
Trace silicon compounds enter the plastic recycling stream from various consumer and industrial sources. In the context of silicon contamination, cosmetic packaging and silicone seals are the primary contributors. These silicones decompose during cracking into volatile siloxanes that distill into the liquid pyrolysis oil fraction, making processing difficult.
Catalyst Poisoning
Transition-metal catalysts used in polymerization and hydrotreating are highly sensitive to trace inorganic impurities. When silicon contamination is present in the monomer feed, it permanently deactivates the active metal sites on the catalyst through the formation of silicon dioxide. This poisoning reduces polymerization efficiency, leading to lower yields and inconsistent molecular weight distribution in the resulting resin.
Resin manufacturers must constantly monitor and filter feedstocks to protect their valuable catalyst beds.
Moulding Defect
Moulders processing resins contaminated with silicon suffer from surface finish problems and structural weaknesses in their finished parts. The low surface energy of silicone impurities causes surface defects like fish-eyes, pitting and poor adhesion of paints or coatings. In thin-wall packaging, silicon contamination can lead to localized stress concentration points that cause premature part failure or pinhole leaks.
Eliminating silicon from the recycling stream is therefore essential for producing high-quality circular polymers that behave like virgin resin, ensuring the mechanical integrity of moulded products.